This species prefers generally open habitats with clumps of trees. It can therefore be found in farmlands and suburban areas, where it comes into regular contact with people. Where persecuted it becomes very wary, but otherwise it is fairly tolerant of human presence. Historically associated with bison herds, it now lands on the back of cattle to glean ticks and insects from them. Large predators such as wolves are commonly followed by black-billed magpies, who scavenge from their kills. The species also walks on the ground, where it obtains such food items as beetles, grasshoppers, worms, and small rodents.

The black-billed magpie is one of the few North American birds that build a domed nest. This nest is made up of twigs and sits near the top of trees. Usually 6-7 eggs are laid. Incubation, by the female only, starts when the clutch is complete, and lasts 16-21 days. The nestling period is 3-4 weeks.

It appears that after the ancestral magpie spread over Eurasia, the Korean population became isolated, at which point the species crossed the Bering Land Bridge and colonized North America, where the two American magpies then differentiated. Fossil evidence indicates that the ancestral North American magpie had arrived in its current range around the mid-Pliocene (3–4 mya) and that the yellow-billed magpie lineage split off rather soon thereafter due to the Sierra Nevada uplift and the beginning ice ages.[3] A comparatively low genetic difference, however, suggests that some gene flow between the black-billed and yellow-billed magpies still occurred during interglacial periods until the Pleistocene.

The adult black-billed magpie is 45–60 cm (18–24 in) long and weighs 145–210 g (5.1–7.4 oz). Males are generally 6–9% larger and 16–20% heavier than females.[4][5] The tail is long and makes up half of the bird's length. Wingspan is about 60 cm (24 in). The bird is black with white shoulders, a white belly, and iridescent dark blue-green wings and tail. There are large white markings on the primaries, clearly visible in flight. The feet and bill are black.

Black-billed magpies range in the north from coastal southern Alaska, central British Columbia, and the southern halves of Alberta, Saskatchewan, and Manitoba, through the Rocky Mountains down south to all the Rocky Mountain states including New Mexico, Colorado, Wyoming, Idaho, and some bordering states as well. The range extends as far east as Minnesota and Iowa, but is thought to be limited further east by high temperature and humidity.[6][7]

Black-billed magpies frequent open country with thickets and scattered trees, especially riparian groves. They can be found near farms and within cities and suburbs. Dry open pine forests are another possible habitat.

The black-billed magpie is an opportunistic omnivore, eating many types of insects, carrion, seeds, rodents, berries, nuts, eggs, and also garbage and food from pets that are fed outside. Chicks are fed animal matter almost exclusively. Magpies typically forage on the ground, usually walking, sometimes hopping, and sometimes scratching with their feet to turn over ground litter. They sometimes land on large mammals, such as moose or cattle, to pick at the ticks that often plague these animals. They often follow large predators, such as wolves, to scavenge or steal from their kills.

Black-billed magpies are also known to make food caches in the ground, in scatter-hoarding fashion. To make a cache, the bird pushes or hammers its bill into the ground (or snow), forming a small hole into which it deposits the food items it was holding in a small pouch under its tongue. It may, however, then move the food to another location, particularly if other magpies are in the vicinity, watching. Cache robbing is fairly common so a magpie often makes several false caches before a real one. The final cache is covered with grass, leaves, or twigs. After this the bird cocks its head and stares at the cache, possibly to commit the site to memory. Such hoards are short-term; the food is usually recovered within several days, or the bird never returns. The bird relocates its caches by sight and also by smell; during cache robbing, smell is probably the primary cue.[8]

Adult black-billed magpie pairs stay together year-round and often for life unless one dies, in which case the remaining magpie may find another mate. Divorces are possible: one South Dakota study found low rates of divorce (8%)[9] but one 7-year study in Alberta found divorce rates up to 63%.[10]

Black-billed magpies nest individually, frequently toward the top of deciduous or evergreen trees. Only the nest tree and its immediate surroundings are defended, and so it is possible for nests to be somewhat clumped in space. When this happens (usually in areas with a limited number of trees or with abundant food resources), a diffuse colony is formed. In this the black-billed magpie is intermediate between the European magpie, whose nests are much more spread out because a large territory is defended around each nest, and the yellow-billed magpie, which is always loosely colonial.

Nests are loose but large accumulations of branches, twigs, mud, grass, rootlets, bark strips, vines, needles, and other materials, with branches and twigs constituting the base and framework. The nest cup is lined with fine rootlets, grass, and other soft material. Nests almost always include a hood or dome of loosely assembled twigs and branches, and usually have one or more side entrances. Nests are built by both sexes over 40–50 days, starting in February (though later in northern parts of the range). Old nests can be repaired and used, or a new nest can be built on top, with older nests thus reaching 120 cm high by 100 cm wide (48 inches high by 40 inches wide). Other bird species, including small hawks and owls, often use old magpie nests.

The breeding season for magpies is generally from late March to early July. They nest once a year, but may re-nest if their first attempt fails early. The female lays up to thirteen eggs, but the usual clutch size is six or seven. The eggs are greenish grey, marked with browns, and 33 mm (about 1.3 inch) long. Only the female incubates, for 16–21 days. The male feeds the female throughout incubation. Hatching is often asynchronous. Hatched young are altricial, brooded by the female but fed by both sexes. They fly 3–4 weeks after hatching, feed with adults for about two months, and then fly off to join other juvenile magpies. Fledging success (usually 3–4 young per nest) is lower than clutch size; this is not an unusual state of affairs in species with asynchronous hatching, as some nestlings often die of starvation.

Black-billed magpies breed for the first time at 1 or 2 years of age. The lifespan of the species in the wild is about four to six years.

The most common calls of this bird are a nasal inquisitive "mag mag mag" or "yak yak yak", uttered in a much higher pitch than the European magpie's. Many other calls also exist, such as begging calls by females to their mate or by young to their parents, and distress calls when seized by predators.

The black-billed magpie often forms loose flocks outside of the breeding season. Dominance hierarchies may develop within such flocks. Dominants can steal food from subordinates. Aggressive interactions also occur at point sources of food. Surprisingly, young males are often dominant over – or maybe just tolerated by – adult males.[11][12]

Like American crows, magpies tend to roost communally in winter. Every evening they fly, often in groups and sometimes over long distances, to reach safe roosting sites such as dense trees or shrubs that impede predator movement, or, at higher latitudes, dense conifers that afford good wind protection.[13] In Canada they arrive at the roosting site earlier in the evening and leave later in the morning on colder days.[14] At the roosting site they tend to occupy trees singly; they do not huddle. They sleep with the bill tucked under the scapular (shoulder) and back feathers, adopting this position sooner on colder nights.[15] During the night they may also regurgitate, in the form of pellets, the undigested parts of what they ate during the day. Such pellets can be found on the ground and then used to determine at least part of the birds' diet.[16]

Like other corvids, black-billed magpies indulge in anting (applying ants onto their plumage) and sun-bathing (back facing the sun, head down, wings drooped and spread wide, tail fanned, back feathers fluffed up). They also belong to that group of birds that scratch their head with their foot over the wing.

Molt begins earlier in males than in females, and in second-year birds than in older ones. Primaries, secondaries, tertials and rectrices are replaced sequentially, so the bird can still fly during molting.

Level flight is relatively slow. Maximum sustained speed is 14 m/s (about 30 miles per hour).[17] Non-flapping phases are often interspersed throughout the flight. Changes in direction can be extremely quick, probably helped by the bird's long tail.

When Lewis and Clark first encountered magpies in 1804 in South Dakota, they reported the birds as being very bold, entering tents or taking food from the hand.[18] Magpies formerly followed American bison herds (from which they gleaned ticks and other insects), as well as the bands of Plains Indians that hunted the bison so they could scavenge carcasses. When the bison herds were devastated in the 1870s, magpies switched to cattle, and by the 1960s they had also moved into the emerging towns and cities of the North American West. Today black-billed magpies remain relatively tame in areas where they are not persecuted. However, they become very wary in areas where they are often shot at or disturbed. Especially during the first half of the 20th century, black-billed magpies were considered detrimental to game-bird populations (they sometimes steal bird eggs) and domestic stock (they sometimes peck at sores on cattle), and were systematically trapped or shot. Bounties of one cent per egg or two cents per head were offered in many states. In Idaho the death toll eventually amounted to an estimated 150,000. In 1933, 1,033 magpies were shot in Washington’s Okanogan valley by two teams of bounty hunters. Many magpies also died from eating poison set out for coyotes and other predators.[19]

If regularly disturbed at the nest, magpie pairs will eventually either move the eggs[20] or abandon the clutch altogether, but in the first instance they will defend the nest aggressively. Interestingly, biologists who have climbed nest trees to measure magpie eggs have reported that the parents recognized them personally on subsequent days and started to mob them, overlooking other people in the vicinity.[21]

Many suburban songbird lovers dislike magpies because of their reputation for stealing eggs, but studies have shown that eggs make up only a small proportion of what magpies feed on during the reproductive season.[22] In England, the European magpie also has a reputation for taking eggs, and yet when density of magpie populations increases, songbird density does not decrease; on the contrary, it increases too.[23]

In the United States, black-billed magpies are protected under the Migratory Bird Treaty Act, but "[a] Federal permit shall not be required to control . . . magpies[] when found committing or about to commit depredations upon ornamental or shade trees, agricultural crops, livestock, or wildlife, or when concentrated in such numbers and manner as to constitute a health hazard or other nuisance ".[24] State or local regulations may limit or prohibit killing these birds as well. The species is not threatened and in some areas it has benefitted from forest fragmentation and agricultural developments. Like many corvids however, it is susceptible to West Nile virus.